1) Field of the Invention
This invention relates to an hermetic high-pressure-resistant gland for optical fibres and has particular but not exclusive application for submerged repeaters for a submarine telecommunications link.
2) Discussion of the Prior Art
Our British patent 2153169 (Gunn 10-6-1-1) describes a high pressure gland for optical fibres and/or electrical conductors for use in a submerged repeater. FIG. 1 of that patent describes a water blocking cable entry extending through a main bulkhead. Within the repeater housing and protected from water by the main bulkhead and the cable entry are individual optical fibre glands described in FIGS. 2 and 4. The main purpose of these individual glands is for gas blocking to ensure gas e.g. Hydrogen does not enter the regenerator compartment of the repeater.
Other patent publications which describe various forms of fibre feed-through glands are 2137375A, 2l9l871A, 2187304A, Euro 0274222 and U.S. Pat. No. 4,345,816. All these prior art glands rely on an adhesive bond between the bare fibre and a surrounding casing to achieve the hermetic seal. Either plastics resin material in 2187304 and U.S. Pat. No. 4,345,816, bonding cement in U.S. Pat. No. 2,191,871, solder in U.S. Pat. No. 2137375, and a so-called glass-to-metal seal in Euro 0274222 formed by a glass solder are cited as the materials which are used.
In U.S. Pat. No. 2,137,375 a liquid filler 13 is filled to ensure that if a pressure is applied it is exerted uniformly around the solder. It is claimed that with such an arrangement it is possible to obtain an optical hermetic fixture structure which inhibits the leakage thereinto of sea water and water vapour by virtue of the self-sealing effect which results from the compression of the solder by the liquid filler, even if sea water pressure is applied to the feed-through in case of a cable fault.
These and other fibre gland designs to date have all employed a solid interface technology for hermeticity. For example, soft solder in contact with the metallised fibre and a plated gland body, or a resin or amalgam to seal on coated or stripped fibres.
Two major problems exist using a solid interface. Firstly, due to differing thermal properties, temperature changes result in high mechanical stresses at the interfaces which can eventually lead to bond breakdown so the gas blocking ability is lost. Secondly, any design which requires the protective coatings to be removed from the fibre is likely to cause damage to the fibre. Major damage leading to breaks should be apparent during production testing, but minor undetected damage may result in a later time-dependent failure in service.
It is an object of the present invention to provide a gland design with improved hermeticity and reliability, and in particular to devise a gland which is suitable for conditions of varying temperature and to provide a satisfactory gas block.